The present invention relates to a color filter to be used for a flat display, such as a liquid crystal display, a plasma display, a field emission display, and an electro luminescence panel, a pigment dispersion composition used for the production thereof, and a production method of the pigment dispersion composition.
Conventionally, a color filter having a coloring layer formed, using a composition with a pigment dispersed has been used as a color separation filter for a flat display, such as a liquid crystal display (LCD), a plasma display (PDP), a field emission display (FED), and an electro luminescence panel (EL). For example, in the case of the LCD, a color filter comprising a coloring layer with an organic pigment dispersed and contained, and a black matrix with a black pigment dispersed and contained is used.
As a pigment dispersion composition for a color filter, following compositions have been used conventionally as ultraviolet ray curing type compositions, and thermosetting type compositions.
(Ultraviolet Ray Curing Type)
                pigment        pigment dispersing agent        binder resin        polyfunctional monomer        photo polymerization initiator        resin having an epoxy group (as needed)(Thermosetting Type)        pigment        pigment dispersing agent        binder resin        resin having an epoxy group        hardening accelerator        
Since a high transmittance and a low light scattering property are required for a color filter of a flat display, a pigment dispersion composition for a color filter should maintain a stable state by dispersion a pigment to a primary particle. A pigment dispersing agent to be used for the above-mentioned pigment dispersion compositions is for dispersing a pigment to a primary particle for stabilizing the state thereof. As examples thereof, those mainly having a pigment dispersing function (Japanese Patent Application Laid Open Nos. 5-611196, 5-247354, 10-300919, or the like), and those having both pigment dispersing property and alkaline developing property (Japanese Patent No 2,665,696, Japanese Patent Application Laid Open Nos. 10-46042, 11-33383, or the like) are known. These pigment dispersing agents have the kind of a contained functional group, and the kind of a monomer limited. In either of the pigment dispersing agents, use of a binder resin having a reactivity or an alkaline developing property is the premise thereof.
As a binder resin having an alkaline developing property, a binder resin with a higher-order structure comprising a block having an acid group and a block without an acid group defined is disclosed (Japanese Patent Application Laid Open No. 5-273411).
Moreover, as a binder resin having a reactivity and an alkaline developing property, one having a reactive group introduced using an unsaturated group and an alicyclic epoxy compound (Japanese Patent Application Laid Open No. 8-262221), one having the introduction position of an ethylenically unsaturated group defined (Japanese Patent Application Laid Open No. 5-19467), and the like are disclosed.
In contrast, as a conventional production method of a pigment dispersion composition, a method of mechanically kneading a pigment, a pigment dispersing agent, a solvent and a binder resin (Japanese Patent Application Laid Open No. 7-13016), a method of mixing a dispersion material of a 0.3 μm or less average particle size produced by processing a pigment, a low molecular weight binder, a solvent, and a dispersing agent with a dispersing machine such as a bead mill, and a three roll mill, and a photosensitive resin composition containing a high molecular weight binder (Japanese Patent Application Laid Open No. 9-134004), and the like are disclosed.
In the case of dispersing an organic pigment to the primary particle state only by a pigment dispersing agent in a pigment dispersion composition for a color filter, although it is stable as a pigment dispersing agent, a problem of the pigment aggregation is involved due to insufficiency of the compatibility of the other resin components such as a binder resin, or the like with the pigment dispersing agent, and a problem is involved in that the residue amount after development in a photolithography step is large due to increase of the pigment dispersing agent amount contained in the pigment dispersion composition, and it gives an adverse effect as an impurity on the display reliability of the display, or the patterning performance is deteriorated.
In order to solve the problems, a binder resin having a pigment dispersing function is used. Since such a binder resin is adsorbed on the organic pigment surface, the addition amount of the pigment dispersing agent can be made smaller, and thus the residue amount after development in a photolithography step can be reduced.
However, in the case of using such a binder resin having a pigment dispersing function, a problem of deterioration of the dispersion stability is involved due to reduction of the addition amount of the pigment dispersing agent. Moreover, since both of a binder resin having a bonding function inherent to a binder resin, and a binder resin having a pigment dispersing function need to be used together, problems are involved in that the bonding function by the binder resin is deteriorated relatively, and in that the compatibility of the two binder resins to be used should be taken into consideration and thus the resin design is difficult.
Moreover, according to the production method of a pigment dispersion composition described above, problems are involved in that the particle size distribution of a pigment dispersion material processed with a bead mill is made larger, and in that the process time for obtaining a desired particle size distribution is long Furthermore, problems are involved in that the temperature can be raised due to continuation of the dispersing process so as to raise the viscosity of the pigment dispersion material so that the viscosity of the final composition cannot be set at a desired viscosity, or in that an excessive pressure amount is generated inside the dispersing machine so that the device is stopped or damaged.
Moreover, even in the case the particle size of the pigment dispersion material dispersed by a bead mill is small, the mixing operation of the resin component and the pigment dispersion material is executed unevenly only by dispersing with a roll mill after mixing with the other resin components by an ordinary agitating machine, and thus a problem is involved in that the resin and the pigment are separated, or a long time is needed for the dispersing operation.